1. Field of the Invention
The present invention relates to furnaces for sintering ceramics, particularly non-oxide ceramics, of which inner walls are lined with heat insulating layers, and carbon heaters to be used in such furnaces. This invention further relates to processes for sintering by using such furnaces and carbon heaters, wherein shaped bodies molded with a mixture of non-oxide ceramic powdery materials and sintering aids are heated at a high temperature under an inert gas atmosphere in the furnace.
2. Description of the Prior Art
Nitride ceramic materials such as silicon nitride (Si.sub.3 N.sub.4), boron nitride (BN), or the like are refractory materials and generally added with 5.about.10% of metal oxides (MeO), such as MgO, Al.sub.2 O.sub.3 or the like, or a mixture of the metal oxides with metal nitrides, as sintering aids to promote the sintering. Further, for example, Si.sub.3 N.sub.4 green bodies before sintering generally have about 40 vol % voids. Now, the mechanism of strength development of the silicon nitride during sintering is accounted as formation of a kind of FRC (Fiber Reinforced Ceramics) wherein .beta.-type silicon nitride needle crystals are dispersed as a reinforcement in glassy phases of metal oxides added as the sintering aids, whereby excellent strength characteristics are developed.
Additionally, if an example is given of Si.sub.3 N.sub.4, shaped bodies thereof are generally fired at a high temperature under an inert atmosphere, particularly, at a temperature of 1,700.degree. C..about.1,900.degree. C. under an nitrogen gas atmosphere. A typical furnace to maintain such a high temperature stable under an inert atmosphere comprises a space for accommodating the ceramic shaped bodies, carbon heaters arranged around the ceramic shaped body in said space and heat insulating layers of carbon fiber mat that cover the inner walls of the furnace, which is further provided with a vacuum port and an inert gas feed opening. The above carbon fiber mat has an extremely large volume porosity, usually 70.about.95 vol. % interstices, that is, resulting in a bulk density averaging about 0.2 g/cc, to ensure its excellent heat insulating properties. Alternatively, particularly when the furnace is relatively of a small size, there may be the case where a carbon cylinder to define the shaped body accommodating space and the graphitic carbon heaters is further arranged on inner side of the carbon fiber mat.
During firing of the Si.sub.3 N.sub.4 in a furnace as mentioned above, the carbon fiber mat having a bulk density of about 0.2 g/cc comes into contact with O.sub.2 and H.sub.2 O remaining in the furnace or a trace of oxygen, oxides or oxynitrides generating from the metal oxide containing Si.sub.3 N.sub.4 shaped bodies at high temperatures, so that carbon fibers in surface layers of the mat undergo an oxidation reaction. Therefore, the carbon fibers disintegrate even though by small bits. As a result, not only heat insulating properties of the mat are gradually deteriorated whereby the life of the furnace is shortened but also characteristics of the sintered body are markedly impaired by the disintegrated carbon fiber dusts that fly and suspend in the furnace and eventually adhere to the high porous Si.sub.3 N.sub.4 shaped bodies during or before sintering, and also by gases such as CO, CO.sub.2 or the like formed by oxidation of the carbon dusts that diffuse and contact with the Si.sub.3 N.sub.4. Namely, when the carbon fiber dusts adhere onto the high porous Si.sub. 3 N.sub.4 shaped bodies before or during sintering as mentioned above, the shaped bodies can draw these carbon fiber dusts inside thereof as the shaped bodies contract during sintering. The drawn-in carbon dusts react with sintering aids and metal oxides, to form CO or CO.sub.2 which comes out to diffuse in the furnace atmosphere and simultaneously the metal oxides are reduced into low melting metals which vaporize. Thus, the metal oxides that are to form a glassy phase matrix are lost particularly in the surface layers, leaving skeltons behind. In the skeltonized state, the Si.sub.3 N.sub.4 sintered bodies no longer have excellent characteristics, such as a high strength, high thermal shock resistance, high abrasion resistance or the like, any longer.
Further, the Si.sub.3 N.sub.4 shaped bodies that contact with CO, CO.sub.2, etc. formed in the furnace repeat the following reactions to lose metal oxides (MeO) rapidly: EQU Si.sub.3 N.sub.4 +MeO+CO.fwdarw.Si.sub.3 N.sub.4 +CO.sub.2 +Me.uparw. EQU CO.sub.2 .fwdarw.CO+O EQU C+O.fwdarw.CO
These reactions accelerate the abovementioned formation of the Si.sub.3 N.sub.4 skelton.
In order to prevent such bad influences of the carbon fiber dusts generated from the insulating layer forming carbon fiber mats, an attempt was made wherein a carbon cylinder was arranged on the inner side of the insulating layer as mentioned above. However, it usually has a wall thickness of about 10 mm, so that if the cylinder having such a high heat capacity is put in the furnace body, an excessively large electric load is naturally applied to the heaters, increasing the consumption of the heaters. Moreover, the manufacture of such a big sized cylinder is cost- and time-consuming that it is economically disadvantageous. Additionally, carbon materials that are denser, on the one hand, are less in self-consumption so that the atmosphere in the furnace can be kept clean and, on the other hand, since such materials have so high a thermal expansion coefficient that they are low in thermal shock resistance and repeated thermal stress, so that a cylinder made thereof develops cracks through which carbon fiber dusts pass to fly, doing harm to surfaces of the Si.sub.3 N.sub.4 sintered body, as described above. In order to prevent the crack development, if a cylinder made of a carbon material having a low thermal expansion coefficient is used, the aforementioned disadvantages caused by the high porosity of the material itself will still not be eliminated.
Furthermore, we the inventors, as a result of continuing assiduous efforts that went into the research of the abovementioned problems and the investigation of the causes, have found that materials of the carbon heaters have a close interrelation and mutually act with the quality of nitride ceramic sintered bodies. Namely, since conventional carbon heaters have been aimed principally at the manufacture at a lowest possible cost as far as their heat generation performance is satisfiable, the purity of the constituting material, i.e., graphite, has been given less consideration, so that those having a carbon content of about three nines, containing impurities such as silicon, iron or the like of about several hundreds of ppm have generally been employed. However, when such a carbon heater is heated at high temperatures, attacks and perforations of the graphite are commenced initiating at the sites of impurities such as silicon, iron or the like contained in the graphite and the carbon disintegrates to fly and eventually adhere to the nitride shaped bodies before or during sintering. Thus, as described above, the skeltonization of the surface layers of the sintered bodies takes place. Simultaneously with it, oxygen, oxides or oxynitrides generating from the shaped bodies adversely enter micropores formed in the heater graphite and react with carbon in the depths, to encroach and disintegrate the skeltons of the graphite, emitting carbon particles, whereby the pores are enlarged until formicary-like pores are formed on the heater members. Thus, the skeltonization due to emitting carbon of the surface layers of the sintered bodies is further promoted to accelerate the degradation of the heaters. Such a heater loses its phase balance as required for a heater material, rendering not only an accurate temperature control impossible but also surface electric current increases locally at poromeric portions, resulting in breakage in an extreme case.
Additionally, other than the above-described phenomena, a problem of a bad influence of the suspending carbon particles upon a thermocouple that functions as an important temperature control has been realized a new. Namely, in temperature measurement in a high temperature nitrogen gas atmosphere at 1,700.degree. C..about.2,000.degree. C., a two-color pyrometer that has usually been applied to high temperatures can hardly expect an accuracy due to fluctuaton, etc. induced by convections of gases in the furnace. Accordingly, in order to prevent nitriding by nitrogen gas of tungsten, generally employed is a W/Re thermocouple that is encapsulated in a molybdenumous protective tube typically enveloping argon gas. However, the molybdenumous protective tube is carbonized, when the suspending carbon particles adhere thereto, to form MoC that is very brittle and different in thermal expansion coefficient from Mo, so that cracks develop after several firing operations. From the cracks, the enveloped argon gas leaks out and nitrogen gas enters instead, whereby the tungsten is nitrided causing a change in an electromotive force that eventually results in loss of its accurate function.